Add Infix To Prefix new algorithm

Author: Hardvan

src/main/java/com/thealgorithms/stacks/InfixToPrefix.java

@@ -0,0 +1,83 @@
+package com.thealgorithms.stacks;
+
+import java.util.Stack;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+public final class InfixToPrefix {
+    private InfixToPrefix() {
+    }
+
+    /**
+     * Converts an infix expression to a prefix expression.
+     *
+     * @param infixExpression the infix expression to convert
+     * @return the prefix expression
+     * @throws Exception if the infix expression has unbalanced brackets
+     */
+    public static String infix2Prefix(String infixExpression) throws Exception {
+        if (!BalancedBrackets.isBalanced(filterBrackets(infixExpression))) {
+            throw new Exception("invalid expression");
+        }
+        StringBuilder output = new StringBuilder();
+        Stack<Character> stack = new Stack<>();
+        // Reverse the infix expression for prefix conversion
+        String reversedInfix = new StringBuilder(infixExpression).reverse().toString();
+        for (char element : reversedInfix.toCharArray()) {
+            if (Character.isLetterOrDigit(element)) {
+                output.append(element);
+            } else if (element == ')') {
+                stack.push(element);
+            } else if (element == '(') {
+                while (!stack.isEmpty() && stack.peek() != ')') {
+                    output.append(stack.pop());
+                }
+                stack.pop();
+            } else {
+                while (!stack.isEmpty() && precedence(element) < precedence(stack.peek())) {
+                    output.append(stack.pop());
+                }
+                stack.push(element);
+            }
+        }
+        while (!stack.isEmpty()) {
+            output.append(stack.pop());
+        }
+
+        // Reverse the result to get the prefix expression
+        return output.reverse().toString();
+    }
+
+    /**
+     * Determines the precedence of an operator.
+     *
+     * @param operator the operator whose precedence is to be determined
+     * @return the precedence of the operator
+     */
+    private static int precedence(char operator) {
+        switch (operator) {
+        case '+':
+        case '-':
+            return 0;
+        case '*':
+        case '/':
+            return 1;
+        case '^':
+            return 2;
+        default:
+            return -1;
+        }
+    }
+
+    /**
+     * Filters out all characters from the input string except brackets.
+     *
+     * @param input the input string to filter
+     * @return a string containing only brackets from the input string
+     */
+    private static String filterBrackets(String input) {
+        Pattern pattern = Pattern.compile("[^(){}\\[\\]<>]");
+        Matcher matcher = pattern.matcher(input);
+        return matcher.replaceAll("");
+    }
+}

src/test/java/com/thealgorithms/stacks/InfixToPrefixTest.java

@@ -0,0 +1,41 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+public class InfixToPrefixTest {
+
+    @ParameterizedTest
+    @MethodSource("provideValidExpressions")
+    void testValidExpressions(String infix, String expectedPrefix) throws Exception {
+        assertEquals(expectedPrefix, InfixToPrefix.infix2Prefix(infix));
+    }
+
+    private static Stream<Arguments> provideValidExpressions() {
+        return Stream.of(
+            Arguments.of("3+2", "+32"),  // Simple addition
+            Arguments.of("1+(2+3)", "+1+23"),  // Parentheses
+            Arguments.of("(3+4)*5-6", "-*+3456"),  // Nested operations
+            Arguments.of("a+b*c", "+a*bc")  // Multiplication precedence
+        );
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideInvalidExpressions")
+    void testInvalidExpressions(String infix, String expectedMessage) {
+        Exception exception = assertThrows(Exception.class, () -> InfixToPrefix.infix2Prefix(infix));
+        assertEquals(expectedMessage, exception.getMessage());
+    }
+
+    private static Stream<Arguments> provideInvalidExpressions() {
+        return Stream.of(
+            Arguments.of("((a+b)*c-d", "invalid expression"),  // Missing closing bracket
+            Arguments.of("a++b", "invalid expression")  // Invalid operator
+        );
+    }
+}